Tungsten has the highest melting point among metals, and has a relatively good electrical conductivity; therefore, it is widely used as an electrode material for which thermal resistance is required. However, when tungsten is used as a cathode material, a substance having a low work function is often added for improved thermionic emission characteristics.
As is shown in Table 1 and Table 2 below, among other additives, ThO2 (thorium oxide) has a high melting point and boiling point, and a relatively low work function. Therefore, W—ThO2 alloys have been used as excellent cathode materials. However, since Th is a radioactive element, there are voices against its use, which have led to the aspirations for materials having superior characteristics to those of conventional W—ThO2 alloys.
For example, Patent Document 1 discloses an electrode for discharge lamps, in which a tungsten alloy having an oxide of Pr, Nd, Sm, or Gd added thereto is used. However, the data disclosed in Patent Document 1 illustrates test results for as short a period as 100 minutes, and alloys exhibiting stable characteristics for a long time in practice are not obtained. Patent Document 2 discloses tungsten sintered bodies to which oxides of Ce, Th, La, Y, Sr, Ca, Zr, and Hf are added. However, no detailed data showing the characteristics of the resultant sintered bodies are described.
TABLE 1oxideoxideoxide workoxide workmeltingboilingfunctionfunctionpointpoint(at 0K)(at 1700K)oxide° C.° C.eVeVThO2305044002.553.07CeO2260032273.23.21Lu2O32490—2.33.26Nd2O32272—2.33.3Sm2O3232535272.83.21Tm2O32400—3.27—Y2O3241543002.03.5La2O3225042002.83.1Pr6O112200—2.83.48Gd2O32340—2.13.29Dy2O32340—2.23.18WO314731837——
TABLE 2metal workmetalmetal meltingfunctionboiling pointpoint(at 0K)metal° C.° C.eVTh548417853.3Ce32578002.6Lu331516613.14Nd312710253.3Sm175210723.2Tm172715453.12Y333714102.954La34549203.3Pr32129352.7Gd323313153.07Dy233514093.09W580033874.52